The process of producing sintered powder metal bodies basically includes the steps of mixing powders of raw materials, compressing the powdery mixture into a green compact, sintering the green compact, and conducting after-treatments such as heat treatment to form a final product. Although the final product may be produced by only the basic process, in many cases the final product are subjected to additional working and/or treatments depending on various applications thereof.
For instance, in order to obtain machine parts with an increased mechanical strength, Japanese Patent Application First Publication No. 1-123005 discloses a process for making sintered powder metal bodies. The process includes the steps of compacting a powder metal mixture into a green compact, sintering the green compact to form a preform, compressing the preform by cold forging, and sintering the compressed preform to form a final product. Specifically, the compressing (cold forging) step includes a first temporary-compressing step and a second regular-compressing step. The preform is coated with a lubricant before being compressed in the first temporary-compression step. Subsequent to being compressed in the first temporary-compressing step, the preform is subjected to a negative pressure so that the lubricant present in fine voids in the porous structure of the preform is evaporated and removed therefrom. Then, the preform is re-compressed in the second regular-compressing step.
In accordance with the proposed process, since the lubricant in the fine voids in the porous structure of the preform is removed, the porous structure is squeezed and the fine voids are eliminated in the second regular-compressing step. As a result, the preform may be compressed into the final product having a relatively high density of approximately 7.4-7.5 g /cm.sup.3 which enhances the mechanical strength of the final product.
Meanwhile, in order to further enhance the mechanical strength of the final product, it will be appreciated to increase a carbon content of the final product, namely, an amount of a graphite powder admixed with a metal powder. However, generally, as the amount of the graphite powder admixed increases, an elongation of a sintered body obtained by sintering a preform which is made from the powdery mixture, decreases and a hardness thereof increases. This causes such a problem that, when the preform is re-compressed, deformability of the preform is reduced whereby the re-compression of the preform cannot be sufficiently achieved.
For example, a pamphlet entitled "The Second Presentation of Developments in Powder Metallurgy" issued on Nov. 15, 1985 by Japan Powder Metallurgy Association, page 90, discloses that a sintered body having a carbon content of 0.05-0.5% has an elongation of 10% and a hardness of Rockwell B Scale (HRB) 83. It is empirically known that, if a sintered body has an elongation of not more than 10% and a hardness of more than 60 HRB, the sintered body cannot be readily re-compressed into the final product. Therefore, there is a demand for obtaining a sintered body having an increased elongation and a reduced hardness and thus excellent deformability.
The present inventors have been devoted themselves to studies to obtain machine parts having a higher mechanical strength which are made from sintered metal. According to the studies by the present inventors, it has been noticed that, in a case where the machine parts are obtained by the steps of temporarily compressing a metal powder to form a preform, sintering the preform to form a sintered body and re-compressing and re-sintering the sintered body, characteristics of the sintered body are the important factors that determine the compressibility in the re-compressing step and mechanical properties of the machine parts to be obtained. Further, the present inventors have recognized that, in order to obtain the mechanically strengthened machine parts, it is necessary a sintered body formed contains a predetermined amount of graphite and has a greater elongation, a less hardness and a good deformability.
The present inventors have earnestly continued the studies. As a result of the present inventors' earnest studies, it has been found that the specific properties of the sintered body, especially the elongation and the hardness which give significant influences on the compressibility of the sintered body, have a close relationship with a density of the preform and a structure of the sintered body, particularly a state of the graphite in the structure of the sintered body.
It is an object of the present invention to provide a sintered powder metal body suitable for fabricating high-strengthened machine parts therefrom, which contains graphite in a predetermined amount and has a greater elongation, a less hardness and a good deformability.
It is a further object of the present invention to provide a process for producing the sintered powder metal body.